An adjustment device is shown, for example, in DE-PS 34 26 967. This patent discloses a shaft having a center bore, such that the spindle of an adjustment mechanism for the variable-pitch propeller is disposed within the bore. At its end that is remote from the pump, the shaft itself has a coupling for introducing the torque. The flange of the multi-part shaft is designed hollow, and its floor area contains a step-down gear transmission as well as a bearing for gear wheels which introduce a force. The force is conducted to the step-down gear transmission within the hollow flange through various axial planes. The torque of the adjustment mechanism is generated in a very complicated fashion by an external electric motor with the intermediate connection of an external differential gear. This type of construction disadvantageously requires a large amount of space.
Another example of an adjustment device is disclosed in JP-A 59-176 498 which includes an adjustment mechanism in which a rotor of an electric motor is mounted in the area of a shaft end along with an intermediate connection of a spindle nut and of a slide piece on the shaft end. During operation, the rotor is moved axially relative to the shaft, and an axial thrust force is generated, which, through a yoke within a hollow flange, transmits an axial motion to an adjustment spindle. Since the rotor must execute its axial motion within its stator, an over-proportionally long stator is required which results in an uneconomical operating behavior. In addition, this proposed solution is very complicated and quite prone to malfunction.